relative clauses - What does the phrase "it has been" mean in the sentence?

I came across a headline on a website yesterday, saying:

"Razer Huntsman Elite is the cheapest it has been in the UK, at £158."

I think I can roughly understand the sentence (Razer Huntsman Elite has reached its lowest price, which is £158 ), but the phrase "it has been" really confused me here.

After searching for a while, I still don't understand the grammatical function of the phrase, even though I've found a pattern like this:

(To be) the [a superlative adjective] + it has been + (usually) [ a
preposition phrase]

Examples:

"The unemployment rate is the lowest it’s been since 1969..."
"Alaska's pink salmon catch is the worst it has been in 40 years."

So, my questions are:

(updated)

what the phrase is doing here in the sentence (its function)? (Resolved. Now I know that the phrase "it has been" has somthing to do with the point of the comparison. It limits the scope of the comparison. Thanks to the authors of the comments).

But new questions arise to me. I add some details that might be more grammatical:

What I'm trying to figure out now is the grammatical relationship between "it has been" and the other part of the sentence. That is, the form of the sentence (how and which part does "it has been" grammatically combined with?). I don't know how to express it more accurately, so let me take a similar but not exact same sentence for example:

He is the kindest person (whom) I have ever met.

In the sentence, I know that "I have ever met" is a relative clause, which modifies the noun "person" before it. A relative clause usually has a relative pronoun preceded, which can be the Object, the Complement or the Subject of the clause, and "whom" is the object of the clause in this case. Those are what I am looking for.

Back to the original sentence again:

Razer Huntsman Elite is the cheapest <something omitted here?> it has been in the UK, at £158.

My revised questions are:

Is "it has been" a relative clause in the sentence? If it is, is there any relative pronoun preceded and omitted?

Thank you